Amphibian model systems will be used to explore several immunologic issues. Emphasis during the coming year will be on thymus-derived (T) cell regulation of humoral responses. We will be continuing to take advantage of important differences seen when one compares immunologic aspects of a urodole, e.g. (Notopthalamus viridescens, the common American newt) and an anuran, e.g. (Xenopus laevis, the South African clawed toad). During the past year we made progress toward developing an understanding of some regulatory events involved in antigen-competition, hapten tolerance and hapten memory in these two model systems. We plan on continuing these studies using both in vivo and in vitro immunization protocols.